In recent years, with the increased focus on cleanliness, surfactants have frequently been used for washing hair and/or cleaning skin. On the other hand, the number of consumers who suffer from hand roughness and dermatitis caused by frequent use of surfactants has also been increasing. In fact, the Ministry of Health, Labour and Welfare's patient census (separated according to disease categories) show that the number of atopic dermatitis patients increased from 224,000 in 1987 to 399,000 in 2009, and that 369,000 patients existed in 2011. This suggests that the number of consumers who suffer daily from damage by surfactants (such as dry-skin feeling, rough skin, skin cracks, eczema, dry-hair feeling, etc.) can be potentially large.
Under such circumstances, amino acid surfactants such as N-acyl-L-glutamic acid triethanolamine, and polyglycerol fatty acid ester surfactants such as polyglyceryl laurate are increasingly used as relatively safer surfactants. As it causes fewer safety problems, soap (fatty acid salt) has been reconsidered. However, medium-chain fatty acids having a carbon chain length of 6 to 10 carbon atoms, such as soaps, are known to be highly irritating. Among the above surfactants, amino acid surfactants are known to be relatively less irritating (low irritation); however, even these surfactants are not completely satisfactory.
Biosurfactants, which are surfactants derived from living organisms, have high biodegradability and high safety and are therefore expected to be used industrially as next-generation surfactants. Among these, sophorolipids, which are a type of glycolipid biosurfactant, are fermentation products that are obtained from fermentation of yeast and known to be highly safe. For example, Patent Literature (PTL) 1 discloses that sophorolipids have a low degree of irritation. However, the degree of low irritation of the sophorolipids disclosed therein is equivalent to that of amino acid surfactants, and further reducing irritation (toxicity reduction) is desired. Furthermore, lactonic sophorolipids, which are a form of sophorolipid, are known to be relatively toxic (Non-patent Literature (NPL) 1).